docs: def get_querystring_for_page(
          request, page_number, querystring_key, default_number=1):
      """Return a querystring pointing to *page_number*."""
      querydict = request.GET.copy()
      querydict[querystring_key] = page_number
      # For the default page number (usually 1) the querystring is not required.
      if page_number == default_number:
          del querydict[querystring_key]
      if 'querystring_key' in querydict:
          del querydict['querystring_key']
      if querydict:
          return '?' + querydict.urlencode()
      return ''